The cement mixer which was driven into the gate of Ireland's parliament building in Dublin with "Toxic Bank Anglo" written on it. Photograph: Stringer/AFP/Getty Images

The full cost of the 2008 banking crisis in Ireland will be laid bare tomorrow when the republic's government is expected to admit that bailing out Anglo Irish Bank will cost at least €30bn (£25.9bn) – equivalent to a fifth of the country's national output.

In a statement that the Irish government hopes will calm markets that have driven the country's borrowing costs to record levels, the finance ministry will concede that it needs to pour another €5bn into Anglo Irish to prevent it collapsing under the weight of bad property loans.

Two years to the day since Ireland had to adopt emergency measures to prop up its banking sector in the wake of the collapse of Lehman Brothers on Wall Street, the Irish government is still trying to stabilise those banks that lent too freely to commercial property developers in the run-up to the crisis.

The near-collapse of the banking sector and the nose-dive in the property market drove the so-called Celtic Tiger into recession and forced the country to the first in the eurozone to adopt an austerity budget. It is now planning a further €3bn of spending cuts and tax increases to help shrink its budget deficit, which at 11.6% of national output is one of the biggest in the eurozone.

The government and its agencies are planning a series of announcements tomorrow that are intended to show how much more help the country's banks need and provide details of how Ireland will manage the costs over the next 10 years. Aside from Anglo, Allied Irish Bank may also need more capital.

The bailout of Anglo Irish is being closely watched by the markets, as ratings agency Standard & Poor's has already warned that if it tops €35bn it could put the country at risk of a further downgrade on its debt rating.

Ireland's borrowing costs have been driven to record levels of close to 6.8% – some 4.5 percentage points higher than Germany's – over fears that its budget deficit will be too difficult for its government to control.

Some €24.5bn of public money has already been promised to Anglo Irish and protesters angry at the scale of the bailout today crashed a cement truck, emblazoned with the words "Toxic Bank Anglo" into the gates of the Irish parliament in Dublin. If reports that a further €5bn may be needed to stabilise the bank are true, this would take the total cost to about €30bn.

The European commission has tried to calm concerns that Ireland might need a Greek-style bailout, although the EU economics commissioner Olli Rehn recently demanded that Ireland outline specific budget measures for the next four years to demonstrate its commitment to keeping its deficit under control.

Anglo Irish Bank is one of the participants in Ireland's national asset management agency (Nama), which is being used to house the most troubled bank loans. A director of Nama, Peter Stewart, today warned of long-term losses for developers in Northern Ireland, where it is set to take over further commercial property loans.

Stewart said that even over 10 years, some of the loans that were granted on land for property development would not recover their value. "Even over such a period of time it is likely that we will see what was previously viewed as potential development land being sold to go back to farm land," Stewart said.

"For those builders, developers and also land traders and speculators who got caught up in the frenzy, unfortunately there is going to be financial pain," he warned.